1. Field of the Invention
This invention relates to a heat-resistant ferritic cast steel member which is excellent in resistance to thermal fatigue and resistance to oxidation and is suitable for parts of the exhaust system of a vehicle such as an exhaust manifold, a flange for an exhaust pipe and the like, and to a method of manufacturing the same.
2. Description of the Prior Art
Conventionally, parts of the exhaust system of a vehicle have been generally made of heat-resistant cast iron such as high-silicon ductile cast iron or Ni-resist cast iron.
Though having excellent casting properties, those heat-resistant cast irons have become insufficient in resistance to heat as the output power of the automotive engines increases and the temperature of exhaust gas increases.
It has been known that ferritic cast stainless steel containing therein 16 to 20 wt % Cr is excellent in resistance to heat. However such ferritic cast stainless steel is poor in resistance to fatigue due to separation of coarse chrome carbide.
For example, in Japanese Unexamined Patent Publication No. 1(1989)-159354, there is disclosed ferritic cast stainless steel having the following composition.
C--0.06 to 0.20 wt %; N--0.01 to 0.10 wt %; Si--0.4 to 2.0 wt %; Mn--0.3 to 1.0 wt %; P--not more than 0.04 wt %; S--not more than 0.04 wt %; Cr--15 to 22 wt %; Nb--0.01 to 2.0 wt %; Ti--0.01 to 0.10 wt %; Mo--0.2 to 1.0 wt %; Ni--0.01 to 1.0 wt %; Y and/or Ce--0.01 to 0.2 wt %; W--0.01 to 1.0 wt %; B--0.001 to 0.01 wt %; V--0.01 to 1.0 wt %; Fe--balance to 100
Though the ferritic cast stainless steel contains boron, the boron content is too small to prevent separation of coarse chrome carbide which adversely affects resistance to thermal fatigue.
When a heat-resistant ferritic cast steel member is cast by conventional sand casting, a core which conforms to the shape of the member is necessary, which results in poor dimensional accuracy and poor yield due to large sinkage, and when burr is generated along the parting line, the burr is hard to chip due to toughness of the material, which greatly deteriorates productivity. If the heat-resistant ferritic cast steel member is cast by use of foamed polystyrene lost model instead of sand casting, carbon enters molten metal when the lost model is substituted by molten metal and sever carburizing phenomenon takes place, whereby carbides separate near the surface of the product and resistance to thermal fatigue and machinability greatly deteriorate.